(1) Field of the Invention
This invention relates to color tone correcting apparatus for converting color component signals forming pixels in an original image into color component signals of desired target colors, thereby to effect a tone correction of the original image. The color component signals to be converted may be those of R (red),G (green) and B (blue) (hereinafter called RGB signals) Of pixels in various video images such as an original image read by a color scanner, or those of C (cyan), M (magenta), Y (yellow) and K (black) (hereinafter called CMYK signals) converted from the RGB signals for plate-making purposes.
(2) Description of the Related Art
Generally, a color tone correction is carried out by correcting color component signals forming pixels in an original image to convert the pixels into color component signals of a target color.
Where, for example, the pixels are formed of RGB signals of the additive primaries, predetermined parameters are applied to the RGB signals of an original image. Specifically, the parameter applied to the R signal is given an increased value to increase the red component of the original image, and the parameter applied to the G signal is given an increased value to increase the green component of the original image. Such parameters are applied to the RGB signals of the original image in converting these signals into RGB signals of a target color to effect a tone correction of the original image. It will be appreciated that the parameters applied in respect of individual pixels in the original image will result in an enormous amount of computation. Thus, tone correction is often carried out using a lookup table which applies predetermined parameters to different values of the RGB signals.
When it is desired to effect a tone correction of only a particular region of the original image, a pointing device such as a mouse is used to designate the particular region. Then the above parameters are applied only to the RGB signals of the designated region.
However, it is a difficult and troublesome operation to designate such a particular geometric region. For example, a change in the color of a red sweater in the original image may be desired while leaving the red color of an apple appearing adjacent thereto unchanged. It is necessary in this case to operate a pointing device such as a mouse along the outline of the sweater to determine a region to be subjected to a tone correction. This results in a disadvantage of an extended time required for tone correction processing.
Under the circumstances, a method has been proposed in which a region in a color space is designated, instead of designating a geometric region as above, and tone correction processing is carried out only for pixels within this region. Preferably, the color space is an HSL color space using, in a three-dimensional coordinate system, hue H, saturation S and lightness L, which are the three attributes of colors perceptible by humans. An apparatus for executing the proposed method converts the values of RGB signals of pixels in an original image into HSL values, and determines whether these values are included in a designated range of HSL color space. The HSL values of the pixels within this region are corrected and converted back to the original RGB signals.
However, such an apparatus requires means (such as a lookup table) for converting the values of RGB signals into HSL values, and an additional lookup table acting as means for converting the HSL values back to the values of RGB signals. The lookup tables must have a sufficient level of precision to render the two conversions reversible. A memory of large capacity is also required for storing these tables. All these result in a costly tone correcting apparatus.
The designation of a particular geometric region or the designation of a particular region in a color space may be made in an original image showing smooth, gradual variations throughout. In this case also, color conversion is effected for the designated region by using the parameters. This entails the disadvantage of producing an unnatural image with colors changing abruptly across boundaries of that region.
On the other hand, numerous methods and apparatus have been proposed for converting RGB signals of an original color image displayed on a color monitor or the like into the subtractive primaries (CMY signals) and black (K signal) for plate-making purposes, and effecting a tone correction of the CMYK signals after the conversion. However, an apparatus simple to operate and assuring a satisfactory image quality has not been put to practical use yet.
A color scanner for use in plate-making, for example, provides a satisfactory image quality. With the plate-making color scanner, RGB signals of an original image are converted into CMYK signals by masking processing, and thereafter a tone correction is effected using its tone correcting function called color correction.
In the masking processing, the RGB signals (Rx, Gx and Bx) of pixels in the original image are converted into CMYK signals (Cx, Mx, Yx and Kx) by means of the following RGB/CMYK converting equations, for example: EQU Cx=.alpha..sub.11 Rx-.alpha..sub.12 Gx-.alpha..sub.13 Bx EQU Mx=-.alpha..sub.21 Rx+.alpha..sub.22 Gx-.alpha..sub.23 Bx EQU Yx=-.alpha..sub.31 Rx-.alpha..sub.32 Gx+.alpha..sub.33 Bx EQU Kx=MAX(Cx,Mx,Yx)
where .alpha.ij (i=1 to 3, j=1 to 3) is a predetermined coefficient (positive value), and MAX (Cx, Mx, Yx) is a function to pick out the greatest of Cx, Mx and Yx.
The respective parts of the Cx, Mx, Yx and Kx signals converted through the above equations may be separated into six types of color signals .DELTA.C, .DELTA.M, .DELTA.Y, .DELTA.R, .DELTA.G and .DELTA.B of C (cyan), M (magenta), Y (yellow), R (red), G (green) and B (blue). These may be expressed as follows: ##EQU1## where .beta.ij (i=1 to 4, j=1 to 6) shows a variable coefficient in each case.
In the color correction, coefficient .beta.ij is adjusted to effect a tone correction of Cx, Mx, Yx and Kx signals.
That is, a control panel connected to a circuit for operating the above equations includes 24 knobs for adjusting coefficient .beta.ij. The operator adjusts each knob to vary coefficient .beta.ij for a desired tone correction.
However, the color correction is difficult to control, and only a highly skilled operator can successfully carry out a desired tone correction. Moreover, where the color the operator desires to correct lies in boundaries of the six hues (C, M, Y, R, G and B), for example, these hues overlap one another in a complex manner. In such an instance, a very high level of technique is required to carry out a tone correction.
A tone correction based on the color correction does not include elements for determining lightness or saturation. This results in the disadvantage that the tone correction is unavailable where, for example, correction of only a light red part in output signals is desired.